This package re-implements measures of ecological diversity from several
other software packages, including
You can install
abdiv from github with:
# install.packages("devtools") devtools::install_github("kylebittinger/abdiv")
Let’s say we’ve surveyed a field and counted the number of plants in each of two sites. We’ve found five species in total, and we’d like to summarize the diversity of the two sampling sites. The diversity within each site is called α-diversity.
Here are the number of plants for each species at site 1 and site 2, represented as vectors.
site1 <- c(2, 5, 16, 0, 1) site2 <- c(0, 0, 8, 8, 8)
The two sites have about the same number of total plants, but the distribution of species is much different. More than half the plants in site 1 belong to a single species, whereas the plants in site 2 are almost evenly distributed across three different species. To get started, let’s look at a few ways to quantify the α-diversity for each sample.
Richness measures the total number of species in each sample.
richness(site1) ##  4 richness(site2) ##  3
The Shannon index measures both the number of species and the evenness of the relative abundance values. Site 2 has fewer species, but each species has the same relative abundance, so the Shannon index is higher.
shannon(site1) ##  0.9365995 shannon(site2) ##  1.098612
Let’s summarize the diversity of site 1 and site 2 using all the
functions available in this library. The full set of within-sample
diversity functions is available as a character vector in
alpha_diversities. The term “α-diversity” means the diversity within a
library(tidyverse) tibble(Site = rep(c("Site 1", "Site 2"), each=5), Counts = c(site1, site2)) %>% group_by(Site) %>% summarize_at(vars(Counts), alpha_diversities) %>% pivot_longer(-Site, names_to = "Measure") %>% ggplot(aes(x=Measure, y=value, color=Site)) + geom_point() + scale_color_manual(values=c("#E64B35", "#4DBBD5")) + coord_flip() + theme_bw()
We can see that site 1 is regarded as more diverse by some measures; it has the most species. For other measures, site 2 is regarded as more diverse; it has the most even distribution of species.
In our documentation, you can find more info on each α-diversity function.
Having assessed the diversity within each sample, we can next ask about the number of shared species between sites. If species are shared, how similar is the distribution across species? There are many ways to quantify the between-sample diversity or β-diversity.
You can think about β-diversity as either the similarity or
dissimilarity between sites. The functions in
abdiv are written in
terms of dissimilarity: similar sites will have values close to zero,
and highly dissimilar sites will have values close to the maximum. This
way of thinking goes along with our intuition about diversity: sites
with greater dissimilarity will exhibit increased diversity if we
consider both sites together.
Let’s look at some examples. The Jaccard distance counts the fraction of species present in only one site. The answer is 3 out of 5, or 0.6.
jaccard(site1, site2) ##  0.6
The Bray-Curtis dissimilarity adds up the absolute differences between species counts, then divides by the total counts. For our two sites, that’s (2 + 5 + 8 + 8 + 7) / 48, or 0.625.
bray_curtis(site1, site2) ##  0.625
Again, we’ll use a vector called
beta_diversities to compute every
dissimilarity measure in the library.
tibble(Measure = beta_diversities) %>% group_by(Measure) %>% mutate(value = get(Measure)(site1, site2)) %>% ggplot(aes(x=Measure, y=value)) + geom_point(color="#4DBBD5") + scale_y_log10() + coord_flip() + theme_bw()
The dissimilarities are generally positive, and they have a range of scales. Some dissimilarity measures range from 0 to 1, while others can go up indefinitely.
As before, you can find more info on each β-diversity function in our documentation.
A phylogenetic tree can be incorporated into diversity measures to add
information about the evolutionary history of species. One of the first
measures in this area was Faith’s phylogenetic diversity. Here, we take
an example from Figure 1 in Faith and Richards (PMID 24832524) to show
how phylogenetic diversity works. The tree is included as
library(ggtree) ggtree(faith_tree, ladderize = F) + geom_tiplab()
If all the species are present, the value of Faith’s phylogenetic diversity (PD) is the sum of the branch lengths. Here, we expect the total branch length to be 5 + 4 + 2 + 4 + 1 + 20 + 5 + 1 + 3 = 45, adding from top to bottom.
Here is how you would calculate Faith’s PD:
faith_pd(c(1, 1, 1, 1, 1), faith_tree) ##  45
If species “a” is missing, we expect Faith’s PD to be reduced by 5, the length of the branch leading to species “a”.
faith_pd(c(0, 1, 1, 1, 1), faith_tree) ##  40
The practice of using phylogenetic information in diversity has been especially popular in microbial ecology, where bacteria are surveyed using the 16S rRNA marker gene. In addition to serving as a fingerprint for bacteria, the gene sequence can be used to build a phylogenetic tree.
In this area, the UniFrac distance is widely used to measure β-diversity
between bacterial communities. We’ll reproduce an example from the
UniFrac paper by Lozupone and Knight (PMID 16332807), which describes
the unweighted UniFrac distance. The tree from Figure 1 is available as
In the example from Figure 1A, we’ve measured two bacterial communities, where the species detected in each are labeled with circles and squares. The communities have no species in common.
ggtree(lozupone_tree, ladderize = F) %<+% lozupone_panel_a + geom_tippoint(aes(shape=SampleID), x=2.6, size=3) + scale_shape_manual(values = c(1, 15)) + scale_x_continuous(limits=c(0, 2.8))
The circle and square communities are all mixed up on the phylogenetic tree. The unweighted UniFrac distance is the fraction of the total branch length where the branch leads to a circle or square, but not both. Here, we would only count the branches leading to the tips of the tree. The length of these branches, added together, is about half the total length for all branches in the tree.
a_circle <- c(1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1) a_square <- c(0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 0) unweighted_unifrac(a_circle, a_square, lozupone_tree) ##  0.5378151
We can increase the UniFrac distance if we rearrange the communities so that all the circles are in the upper part of the tree and all the squares are in the lower part. In the paper, this is shown in Figure 1B.
ggtree(lozupone_tree, ladderize = F) %<+% lozupone_panel_b + geom_tippoint(aes(shape=SampleID), x=2.6, size=3) + scale_shape_manual(values = c(1, 15)) + scale_x_continuous(limits=c(0, 2.8))
With this arrangement, the species in each community are phylogenetically very different. Except for the root, each branch of the tree leads uniquely to a species present in square or circle, and never to a species present in both. Therefore, the UniFrac distance is close to 1.
b_circle <- c(1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0) b_square <- c(0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1) unweighted_unifrac(b_circle, b_square, lozupone_tree) ##  0.9747899
Now that we’ve introduced you to the functions in this package, we should probably tell you how to use them in your work. Your data are likely to be in one of two formats: a long-format data frame, or a matrix of counts. We’ll tackle the long format first.
First, we’ll arrange the data from the plant survey example in long format. We’ll also add a third site to help with the demonstration.
site3 <- c(15, 1, 4, 2, 2) plants <- tibble( Site = rep(c("Site 1", "Site 2", "Site 3"), each = 5), Species = rep(letters[1:5], times = 3), Counts = c(site1, site2, site3) ) plants ## # A tibble: 15 x 3 ## Site Species Counts ## <chr> <chr> <dbl> ## 1 Site 1 a 2 ## 2 Site 1 b 5 ## 3 Site 1 c 16 ## 4 Site 1 d 0 ## 5 Site 1 e 1 ## 6 Site 2 a 0 ## 7 Site 2 b 0 ## 8 Site 2 c 8 ## 9 Site 2 d 8 ## 10 Site 2 e 8 ## 11 Site 3 a 15 ## 12 Site 3 b 1 ## 13 Site 3 c 4 ## 14 Site 3 d 2 ## 15 Site 3 e 2
If we want to compute α-diversity, we can group by
Site and create a
new column with the answer. The α-diversity functions take the number of
counts for each species as an argument.
plants %>% group_by(Site) %>% summarize(Richness = richness(Counts)) ## # A tibble: 3 x 2 ## Site Richness ## <chr> <int> ## 1 Site 1 4 ## 2 Site 2 3 ## 3 Site 3 5
If you want to cover more than one α-diversity measure, you can use
plants %>% group_by(Site) %>% summarise_at(vars(Counts), c("shannon", "invsimpson")) ## # A tibble: 3 x 3 ## Site shannon invsimpson ## <chr> <dbl> <dbl> ## 1 Site 1 0.937 2.01 ## 2 Site 2 1.10 3 ## 3 Site 3 1.14 2.30
The old school way to compute α-diversity is to arrange your data in a
matrix format, with rows representing different sites or “observations”,
and columns representing species. This is how you would do it with the
vegan package, for example. Here is our data in matrix format:
plants_matrix <- matrix( c(site1, site2, site3), nrow=3, byrow=TRUE, dimnames = list(c("Site 1", "Site 2", "Site 3"), letters[1:5])) plants_matrix ## a b c d e ## Site 1 2 5 16 0 1 ## Site 2 0 0 8 8 8 ## Site 3 15 1 4 2 2
In matrix format, we can use the
apply() function from base R to get
the diversity for each site.
apply(plants_matrix, 1, shannon) ## Site 1 Site 2 Site 3 ## 0.9365995 1.0986123 1.1389489
Here is the same example using
vegan::diversity(plants_matrix) ## Site 1 Site 2 Site 3 ## 0.9365995 1.0986123 1.1389489
The long format is friendlier to workflows using functions in the
tidyverse, but the matrix format has some advantages. The matrix
format ensures that every sample has a valid count value for every
species considered; the matrix contains values of zeros for species not
observed. This quality is not critically important for alpha-diversity
measures, but it is important for computing β-diversity.
For β-diversity, we recommend proceeding via the matrix format. If your
data is in long format, the
usedist package has a function to convert
to a numeric matrix.
usedist::pivot_to_numeric_matrix(plants, Site, Species, Counts) ## a b c d e ## Site 1 2 5 16 0 1 ## Site 2 0 0 8 8 8 ## Site 3 15 1 4 2 2
To create a distance matrix between samples, we can employ the
dist_make() function from
usedist::dist_make(plants_matrix, bray_curtis) ## Site 1 Site 2 ## Site 2 0.6250000 ## Site 3 0.6666667 0.6666667
One practical issue with phylogenetic diversity is to make sure that the order of the species in your vectors matches the order of the tree. If your vector is not named, and you’re sure that the order matches, you can use the phylogenetic diversity functions as above. To take the example from Faith’s phylogenetic diversity, let’s say we had a vector representing 10 counts of species “d” and 12 of species “e”.
faith_pd(c(0, 0, 0, 10, 12), faith_tree) ##  29
If your vector is named, the names will be automatically used to match the vector with the tree. Missing names are filled in with zero counts.
faith_pd(c(d=10, e=12), faith_tree) ##  29
If your vector is not named, or if you’re not totally sure that the order of species in your vector matches the order of species in the tree, you can pass in the species labels for your vector as an additional argument.
faith_pd(c(10, 12), faith_tree, c("d", "e")) ##  29
This last approach is useful for a
tidyverse workflow, where the
vectors are not named and the species labels are found in a separate
column. Here is an example for data in long format, where the species
labels are in the column, “Species”:
plants %>% group_by(Site) %>% summarize(FaithPD = faith_pd(Counts, faith_tree, Species)) ## # A tibble: 3 x 2 ## Site FaithPD ## <chr> <dbl> ## 1 Site 1 40 ## 2 Site 2 38 ## 3 Site 3 45
The rules are the same for phylogenetic β-diversity functions. The
dist_make() function from
usedist will pass along additional keyword
arguments to the distance function, so you can give the tree and species
dist_make(). Here’s an example from the plants matrix.
usedist::dist_make(plants_matrix, unweighted_unifrac, faith_tree) ## Site 1 Site 2 ## Site 2 0.2666667 ## Site 3 0.1111111 0.1555556
Now, we’ll re-order the species in the data matrix to see how things work.
species_reorder <- c("d", "e", "a", "b", "c") plants_matrix_reorder <- plants_matrix[,species_reorder] plants_matrix_reorder ## d e a b c ## Site 1 0 1 2 5 16 ## Site 2 8 8 0 0 8 ## Site 3 2 2 15 1 4
The column names of the matrix are automatically added to vectors extracted from the matrix, so we get the same result as before.
usedist::dist_make(plants_matrix_reorder, unweighted_unifrac, faith_tree) ## Site 1 Site 2 ## Site 2 0.2666667 ## Site 3 0.1111111 0.1555556
If the column names are missing, first of all, you’re probably in trouble. But if you do have the species labels stored in a separate vector, you can pass them along to the distance function. Let’s remove the column names from our matrix to check this out.
plants_matrix_reorder_nonames <- plants_matrix_reorder colnames(plants_matrix_reorder_nonames) <- NULL plants_matrix_reorder_nonames ## [,1] [,2] [,3] [,4] [,5] ## Site 1 0 1 2 5 16 ## Site 2 8 8 0 0 8 ## Site 3 2 2 15 1 4
Now, we can pass the species labels to
dist_make() and verify that we
get the correct answer.
usedist::dist_make( plants_matrix_reorder_nonames, unweighted_unifrac, faith_tree, species_reorder) ## Site 1 Site 2 ## Site 2 0.2666667 ## Site 3 0.1111111 0.1555556
Please don’t hesitate to reach out via email or file an issue if you need support when using this library.
The answer here is slightly different than that in the paper. See
faith_tree documentation for further explanation.
abdiv are authored by the same
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